Conventional antennas often have linear dimensions comparable to the wavelength of the radiation being received or transmitted. For example, a typical radio transmitter uses a dipole antenna whose length is about one-half the wavelength of the waves being transmitted. Such an antenna length provides for efficient coupling between the antenna's electrical driver and the radiation field.
However, antennas having linear dimensions comparable to the radiation wavelength are not practical in all situations. In particular, cellular telephones and handheld wireless devices are small. Because such devices provide limited space for antennas, it would be advantageous to equip them with miniaturized antennas. Unfortunately, simply reducing antenna size without deviating from conventional principles leads to small antennas that couple inefficiently to the radiation at the wavelengths typically used in cellular telephones and handheld wireless devices.
U.S. Pat. No. 6,661,392, which issued to Isaacs et al. on Dec. 9, 2003, describes an antenna that resonantly couples to external radiation at communication frequencies even with linear dimensions much smaller than one-half the radiation wavelength. Due to the resonant coupling, the antenna is very sensitive to the radiation.
The antenna includes a resonant object formed of a special material, such as a manmade metamaterial, whose electrical permittivity or magnetic permeability has, in effect, a negative real part at microwave frequencies. One or more sensors located adjacent to or in the object measure an intensity of an electric or a magnetic field therein.
Although antennas based on such special materials have promise, improvements in bandwidth and waveguide coupling efficiency are needed in order for the performance of such antennas to be improved to the fullest possible extent.